1. Field of the Invention
The present invention relates to a method of ultrasonically grinding surfaces of a workpiece immersed in a liquid by radiating ultrasonic energy from an ultrasonic vibrator into the liquid.
2. Description of the Prior Art
Die-cast products such as machine parts have burrs, surface irregularities, and small scratches on their surfaces immediately after they are die-cast. The die-cast products are finalized when these surface imperfections are ground off.
Japanese laid-open patent publication No. 55-90261 discloses a method of finishing a workpiece by removing burrs having a height of about 100 .mu.m from its surfaces or smoothing out relatively large surface irregularities. According to the disclosed method, the workpiece is immersed in a processing liquid containing abrasive grains, and ultrasonic energy is applied to the processing liquid. More specifically, the processing liquid is supplied to a processing tank or chamber equipped with an ultrasonic vibrator, and ultrasonic energy is radiated from the ultrasonic vibrator into the processing liquid in the processing tank to develop cavitation in the processing liquid. The abrasive grains are brought into violent collision with the workpiece surfaces by shock waves that are generated when the cavitation collapses in the processing liquid, thereby grinding the workpiece surfaces.
However, as it has turned out, the disclosed method fails to develop sufficient cavitation when ultrasonic energy is radiated into the processing liquid, and hence the workpiece surfaces cannot fully be ground by the abrasive grains.
Another known method of smoothing out or removing small surface irregularities or scratches from workpiece surfaces is an electrolytic grinding process. According to the electrolytic grinding process, a workpiece such as a metal or semiconductor component and an electrode are immersed as an anode and a cathode, respectively, in an electrolytic solution which may be a mixture of concentrated phosphoric acid and sulfuric acid, and a voltage is applied between the anode and the cathode to carry out electrolysis. The surfaces of the workpiece are dissolved by the electrolysis, smoothing out small surface irregularities or scratches having a height or depth ranging from 0.1 to several .mu.m, so that the surfaces are made glossy. However, oxygen produced when the workpiece surfaces are dissolved is applied as bubbles to the workpiece surfaces and covers the workpiece surfaces. As the electrolysis progresses, the oxygen bubbles cover the workpiece surfaces so much that they prevent the workpiece from contacting the electrolytic solution. Then, the electrolysis fails to go on, and the workpiece surfaces are no longer smoothed.